The mere registration of a name means nothing.
Trademarks are quite different from copyright, where proof of authorship is all; given the relative youth of both sites, I wouldn't rely on a legal remedy if I were twitvid.io. If nothing else, one would have to ask whether the cost of defending the mark would exceed the cost to buyout the .com, and indeed if/why not this was budgeted for.
I'm just skeptical that can be done when both services launched within days of each other. Seems like not enough data to draw any meaningful conclusions.
If bing.io had launched a few days before ms unveiled bing.com do you think they'd have a hope in hell?
I'm not saying what they did is right, as it clearly isn't, but I agree with axod; Twitvid.io should just change their name and move on. They're going to fight a losing battle otherwise.
It's obvious that twitvid.com would become a competitor, and due to the service they both copied - twitpic.com - would look very similar to twitvid.io.